U.S. patent application number 10/587181 was filed with the patent office on 2007-12-13 for wireless communication system capable of switching protocol.
Invention is credited to Akira Maeki, Masayuki Miyazaki, Masaaki Shida, Isao Shimokawa.
Application Number | 20070286102 10/587181 |
Document ID | / |
Family ID | 34917835 |
Filed Date | 2007-12-13 |
United States Patent
Application |
20070286102 |
Kind Code |
A1 |
Shimokawa; Isao ; et
al. |
December 13, 2007 |
Wireless Communication System Capable of Switching Protocol
Abstract
A wireless communication system comprising a management server,
base stations, and a plurality of node terminals, wherein a
wireless communication protocol to be applied is switched
dynamically based on such indices as but error, packet error rate,
the number of times of retransmitting packets, throughput/power
consumption that may occur between each base station and nodes.
Inventors: |
Shimokawa; Isao; (Kokubunji,
JP) ; Miyazaki; Masayuki; (Tokyo, JP) ; Maeki;
Akira; (Kokubunji, JP) ; Shida; Masaaki;
(Hachioji, JP) |
Correspondence
Address: |
REED SMITH LLP
3110 FAIRVIEW PARK DRIVE, SUITE 1400
FALLS CHURCH
VA
22042
US
|
Family ID: |
34917835 |
Appl. No.: |
10/587181 |
Filed: |
March 5, 2004 |
PCT Filed: |
March 5, 2004 |
PCT NO: |
PCT/JP04/02822 |
371 Date: |
April 5, 2007 |
Current U.S.
Class: |
370/310 |
Current CPC
Class: |
H04W 88/06 20130101;
H04W 52/0206 20130101; H04W 92/10 20130101; H04W 80/00 20130101;
H04W 36/14 20130101; H04W 48/18 20130101; H04W 52/0219 20130101;
H04W 52/0216 20130101; H04W 74/08 20130101; H04W 74/006 20130101;
Y02D 30/70 20200801 |
Class at
Publication: |
370/310 |
International
Class: |
H04B 7/00 20060101
H04B007/00 |
Claims
1. A wireless communication system comprising a plurality of node
devices each communicative with a base station, using one of a
plurality of wireless communication protocols, and the base station
which communicates with the plurality of node devices, wherein each
of said node devices and said base station carry out communication,
using a wireless communication protocol selected based on an
evaluation of efficiency of communication between the node devices
and the base station.
2. The wireless communication system according to claim 1, wherein
each of said node devices evaluates said efficiency of
communication based on a retransmit count of transmission
packets.
3. The wireless communication system according to claim 1, wherein
said base station evaluates said efficiency of communication based
on the number of node devices under management of the base
station.
4. The wireless communication system according to claim 1, wherein
said base station evaluates said efficiency of communication, using
packet transmit counts from said plurality of node devices under
management of the base station and receive counts of packets
transmitted from the node devices.
5. The wireless communication system according to claim 1, wherein
said base station evaluates said efficiency of communication based
on error rates for packets received from said plurality of node
devices.
6. The wireless communication system according to claim 1, further
comprising a management server connected to said base station,
wherein said management server evaluates said efficiency of
communication based on the error rates for packets transmitted from
said plurality of node devices and received by the base station or
a combination of the packet transmit counts at the plurality of
node devices under management of said base station and the receive
counts of packets from the node devices at said base station.
7. The wireless communication system according to claim 1, wherein
said efficiency of communication is the amount of data transfer per
power consumption for communication at said node devices.
8. A node device communicative with a base station by selectively
applying one of a plurality of wireless communication protocols,
comprising: a wireless unit; a communication processing unit; and a
wireless communication protocol selecting unit, wherein packet
transmission from said wireless unit to said base station is
controlled using a wireless communication protocol selected based
on an evaluation of efficiency of communication with said base
station.
9. The node device according to claim 8, wherein said communication
processing unit detects a retransmit count of transmission packets,
and said wireless communication protocol selecting unit evaluates
said efficiency of communication based on the retransmit count
detected, and selects a wireless communication protocol to be
used.
10. The node device according to claim 8, wherein said wireless
communication protocol selecting unit selects said wireless
communication protocol based on information indicative wireless
communication protocol switching received from said base
station.
11. The node device according to claim 8, wherein said efficiency
of communication is the amount of data transfer per power
consumption for communication at the node device.
12. A base station communicative with a plurality of node devices
by selectively applying one of a plurality of wireless
communication protocols, comprising: a wireless unit; a
communication processing unit; and a wireless communication
protocol selecting unit, wherein said wireless unit receives
packets from said node devices, using a wireless communication
protocol selected based on an evaluation of efficiency of
communication with said node devices.
13. The base station according to claim 12, further comprising a
database unit in which a table for evaluating said efficiency of
communication is stored, said table correlating applicable wireless
communication protocols and the number of node devices under
management of the base station, wherein said communication
processing unit detects the number of node devices under management
of the base station, selects one of said wireless communication
protocols based on said table and said detected number of node
devices, and instructs said wireless communication protocol
selecting unit to switch to the selected protocol.
14. The base station according to claim 12, further comprising: a
database unit in which a table correlating applicable wireless
communication protocols and efficiency of communication is stored,
wherein said wireless communication protocol selecting unit refers
to said table and selects one of said wireless communication
protocols based on the evaluation of efficiency of
communication.
15. The base station according to claim 14, wherein said
communication processing unit evaluates said efficiency of
communication based on receive counts of packets from said
plurality of node devices at the base station and packet transmit
counts from the plurality of node devices, notified from said
plurality of node devices.
16. The base station according to claim 12, further comprising: a
communication interface for communicating with a management server,
wherein the number of node devices under management of the base
station or the receive counts of packets transmitted from said
plurality of node devices and received by the base station and the
packet transmit counts from the plurality of node devices, notified
from said plurality of node devices, are notified to said
management server through the communication interface, and said
wireless communication protocol is selected based on information
for wireless communication protocol switching notified from said
management server.
17. The base station according to claim 12, wherein information of
said selected wireless communication protocol is transmitted as
information for wireless communication protocol switching to said
plurality of node devices.
18. The base station according to claim 12, wherein said efficiency
of communication is the amount of data transfer per power
consumption for communication at the node devices.
Description
TECHNICAL FIELD
[0001] The present invention relates to an apparatus for switching
among wireless communication protocols to be used between a base
station and a node.
BACKGROUND ART
[0002] As shown in FIG. 1, a conventional general wireless
communication system (mobile communication system) is comprised of
a mobile communication switch 101, a switch 102, a location
registering server 103, base stations (BSs) 104 and 105, and mobile
terminals (nodes) 106 and 107 to be used by users. Each node is
connected to a base station BS via a radio channel and each base
station is connected to the mobile communication switch 101 via a
wired line. To enable connection to an ordinary telephone line, the
mobile communication switch 101 is connected to a fixed-line
telephone network via the switch 102. Further, the location
registering server 103 is connected to the mobile communication
switch 101 to perform handover control between base stations upon
incoming a call to each node.
[0003] In a mobile communication system, it is important in system
design to determine a cellar zone and a wireless communication
protocol between nodes and each base station in order to reduce
radio interference between base stations and between nodes. Here,
the cellar zone refers to a coverage range of node management for
each base station. The cellar zone has a close relationship with a
transmission power and a receiving sensitivity of each base station
and nodes. In general, each base station is disposed to minimize
the radio interference between the base station and neighboring
base stations.
[0004] In each cellar zone, a plurality of nodes located within the
zone (cell) communicate with the base station, using a
predetermined wireless communication protocol. In a mobile
communication system, a specific wireless communication protocol
optimum for the system's requirement specifications is adopted. At
present, various wireless communication protocols with which each
base station performs wireless communication with a plurality of
nodes are proposed. Each of these protocols has its own advantages
and disadvantages. Therefore, a specific wireless communication
protocol defined based on the requirement specifications does not
always assure a high efficiency (high throughput and low power
consumption) in various situations under which a mobile
communication system has to work.
[0005] FIG. 3 illustrates the features of ALOHA, CSMA, and TDMA as
typical wireless communication protocols for packet
communication.
[0006] The ALOHA is a multiple access packet communication method
in which a base station or a node transmits a packet immediately
when a call has been originated, as illustrated in Figure (A). In
the ALOHA method, as the number of nodes located in a cell
increases, the probability of packet loss increases because a
packet transmitted from each node will possibly collide with a
packet transmitted from another node, whereby an overall throughput
of the system decreases.
[0007] The CSMA is a multiple access packet communication method in
which a base station or a node detects the transmission situation
of packets from the other base stations or nodes by carrier sense
control and determines whether to transmit a packet when a call has
been originated, as illustrated in Figure (B). Thus, a CSMA system
has features of a lower probability of packet loss and a higher
overall throughput of the system.
[0008] An ALOHA system does not performs the above-mentioned
carrier sense control. Accordingly, the ALOHA system is lower in
power consumption than the CSMA system and contributes to longer
lifespan of a battery of each node, provided that power consumption
for packet retransmission required when a packet loss occurs is not
taken in consideration. Therefore, if the number of nodes in a cell
is small, the ALOHA system is more favorable than the CSMA system
in terms of power consumption. Inversely, if the number of nodes is
great, the CSMA system with less packet loss is more favorable than
the ALOHA system.
[0009] The TDMA is a method in which a period of communication
(frame period) between a base station and nodes is divided into a
plurality of time slots and the base station and each node transmit
packets, using pre-allocated time slots, as illustrated in Figure
(C). A TDMA system has a complex system structure because a high
time precision is required as the whole system and each node must
be synchronized in time with the base station so that the nodes
transmit signals in time slots allocated to them. Although no
packet loss occurs in the TDMA method theoretically, the TDMA
system has a high overall power consumption because control signals
for maintaining such a high time precision must be transmitted and
received between the base station and each node.
[0010] Heretofore, proposals have been made for switching to an
alternative mobile communication system having a higher efficiency
of data transmission adaptively for a communication environment
such as the number of nodes in a cell, noise level, or throughput,
for example, in Japanese Patent Publication No. 2002-64871 and
Japanese Patent Publication No. 2002-247049.
[0011] If a communication environment is evaluated only in terms of
throughput, there would be no need to take account of the number of
packet retransmission (retransmit count) at each node. When a
packet collision occurs, if succeeded in transmission eventually by
packet retransmission, the retransmit count has no influence on
throughput. However, because the power consumption of a
transmitting node increases if packet retransmission occurs, such
evaluation only based on throughput is not proper for a mobile
communication system requiring to suppress the power consumption of
a node as low as possible. On the other hand, if a communication
environment is evaluated only in terms of noise level, it would be
difficult to determine a threshold value. Further, since control
packets for confirming a noise level must be communicated,
additional power is consumed in the base station and each node by
transmitting and receiving the control packets.
[0012] An object of the present invention is to provide a wireless
communication system that can selectively apply a wireless
communication protocol adapted for the communication environment
between a base station and a node device (mobile terminal).
[0013] Another object of the present invention is to provide a base
station and a node device (mobile terminal) capable of switching to
a wireless communication protocol to be applied according to the
communication environment in a wireless communication system.
DISCLOSURE OF THE INVENTION
[0014] In order to achieve the above objects, in the present
invention, a wireless communication protocol to be used for
communication between a base station and node devices (mobile
terminals) is selected based on a communication efficiency
evaluation which will be described below. Here, "Ef" is defined in
Equation (1) as an index of evaluating the efficiency of a wireless
communication system. Ef=Overall throughput of system/Overall power
consumption of system (1)
[0015] Ef defined by Equation (1) indicates a higher value as the
overall throughput of system is higher and the overall power
consumption of system is lower. In the present invention, the
values of Ef are evaluated in relation to the number of nodes in
each cell. The evaluated Ef values are stored beforehand in a table
to be held in any of a management server, a base station, and a
node, thereby to perform selective switching between or among
wireless communication protocols based on the table values.
Parameters that can be used to approximately evaluate the Equation
(1) are, for example, retransmit count, bit error rate (BER),
packet error rate (PER), etc. Thus, in actual application,
selective switching between or among wireless communication
protocols is performed by using these parameters.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a schematic diagram of a conventional wireless
communication system.
[0017] FIG. 2 is a schematic diagram of a wireless communication
system according to the present invention.
[0018] FIG. 3 shows a set of diagrams for comparing the features of
ALOHA, CSMA, and TDMA methods.
[0019] FIG. 4 shows comparison results of the overall power
consumptions of the systems of ALOHA, CSMA/CA, and TDMA
methods.
[0020] FIG. 5 shows comparison results of the overall throughputs
of the systems of ALOHA, CSMA/CA, and TDMA methods.
[0021] FIG. 6 shows comparison results of the overall
throughputs/overall power consumptions of the systems of ALOHA,
CSMA/CA, and TDMA methods.
[0022] FIG. 7 shows an example of a table for protocol switching
according to the number of nodes.
[0023] FIG. 8 shows an example of a table for protocol switching
according to the number of nodes and the amount of data transfer
per power consumption.
[0024] FIG. 9 is a structural diagram of a node device to which the
present invention is applied.
[0025] FIG. 10 is a structural diagram of a base station to which
the present invention is applied.
[0026] FIG. 11 is a structural diagram of a management server to
which the present invention is applied.
[0027] FIG. 12 is a flowchart illustrating basic system operation
of a conventional node device.
[0028] FIG. 13 is a flowchart illustrating basic system operation
of a conventional base station.
[0029] FIG. 14 is a flowchart illustrating one embodiment of basic
operation of a node device according to the present invention.
[0030] FIG. 15 is a flowchart illustrating another embodiment of
basic operation of a node device according to the present
invention.
[0031] FIG. 16 is a flowchart illustrating one embodiment of basic
operation of a base station according to the present invention.
[0032] FIG. 17 is a flowchart illustrating further another
embodiment of basic operation of a node device according to the
present invention.
[0033] FIG. 18 is a flowchart illustrating another embodiment of
basic operation of a base station according to the present
invention.
[0034] FIG. 19 is a flowchart illustrating one embodiment of basic
operation of a management server according to the present
invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0035] Embodiments of the present invention will now be described
with reference to the drawings.
[0036] In the present invention, the value Ef for the Equation (1)
indicative of a system efficiency evaluation will be higher as the
overall throughput of the system is higher and the overall power
consumption of the system is lower. This evaluation value Ef is
important especially for a system, for example, a Sensor Net, for
which lower power consumption is required.
[0037] The Sensor Net is a generic term meaning a network system
where a network is formed by deploying a large number of small
nodes, each equipped with a sensor, a power supply, and a wireless
communication function, and sensed information is transfer to a
management system or a Web site on the Internet via the network.
The Sensor Net is expected to be applied in fields such as building
monitoring, environment monitoring, security, and commodity
distribution, etc.
[0038] Here, as the power supply of a node, a battery or self-power
generator is often used and required in many applications. As the
power supply, for example, if a battery is used, frequent
replacement of the battery is problematic in maintenance and
operation of a node. In the case of self-power generator, its power
supply capacity is often small and operation with lower power
consumption is inevitable. Lower power consumption is important
especially for the wireless communication function that consumes a
large portion of power consumption of a node.
[0039] Meanwhile, each node has to perform efficient wireless
transmission of sensed information to a base station or the
network. Therefore, each node is required to perform efficient data
transmission with reduced power consumption. As an index for
realizing this, the value of Ef indicating the amount of data
transfer per power consumption which is expressed by Equation (1)
becomes important. For an inefficient system with a lower index
value of Ef, the lifespan of the battery of a node is very short
and the range of system applications is narrowed. That is, for a
system like a Sensor Net, it is important to build a high
efficiency wireless system having a higher index value of Ef.
[0040] The ALOHA, CSMA, and TDMA methods are compared here as
wireless communication protocols being selectable in the present
invention.
[0041] Simulation results of power consumptions of the overall
systems are shown in FIG. 4, wherein the power consumptions vary
depending on the number of nodes managed by base stations.
Simulation results of the overall throughputs of these systems are
shown in FIG. 5. FIG. 6 shows simulation results evaluated by
Equation (1). From the simulation results of FIG. 6, it is apparent
that the efficiencies of the systems vary depending on the number
of nodes to be managed.
[0042] The index value of Ef expressed by Equation (1) has a close
relationship with the number of nodes to be managed by the base
station. As the number of nodes increases, the number of requests
for packet transmission (offered load) increases proportionally and
the probability of packet loss at the nodes increases. Thus, it is
possible to evaluate Equation (1) with the number of nodes in
advance, providing any of a management server, a base station, and
a node with a table indicating the results of the evaluation, and
select a wireless communication protocol based on the values held
in this table.
[0043] The index value of Equation (1) can be replaced by, for
example, retransmit count, BER, or PER. When mutual interference
between nodes increases, noise power becomes larger, resulting in
an increase of BER or PER. Thus, the efficiency of each system can
be evaluated in terms of BER or PER. Moreover, since the retransmit
count increases with an increase of PER, the efficiency of each
system can be also evaluated in terms of retransmit count. To
implement these evaluations, conceivable three possible patterns
are node initiative, basic station initiative, and management
server initiative.
[0044] Retransmit count, BER, and PER can be measured at each of
nodes and base stations. By transmitting retransmit count, BER, and
PER from a base station to a management server, it is also possible
to measure these parameters at the management server. Therefore, a
wireless communication protocol to be applied can be changed by any
of nodes, base stations, and the management server, taking an
initiative role.
[0045] However, as implied from the comparison of the ALOHA, CSMA,
and TDMA, switching to the TDMA must be performed by a base station
initiative. This is because, in the case of the TDMA system, the
base station must allocate time slots to each node and each node
cannot perform protocol switching autonomously. On the other hand,
switching to the CSMA or ALOHA may be carried out by either a node
or the base station. This is because the essential difference
between the CSMA and the ALOHA is whether carrier sense is
performed or not, and the overall operation of the system can be
maintained even if, for example, a node performs protocol switching
autonomously. In the following, embodiments will be described with
regard to three types of wireless communication protocol switching
different in initiative device.
EMBODIMENT 1
[0046] As a first embodiment, a method of node device initiative
wireless communication protocol switching is described. Here, ALOHA
and CSMA are prepared as switchable wireless communication
protocols. Both the ALOHA and CSMA do not require for a plurality
of node devices to switch to an alternative one at the same time in
the whole system. The ALOHA and CSMA methods allow each node device
to carry out scheduling by itself without being instructed from the
base station.
[0047] A wireless communication system shown in FIG. 2 is comprised
of a management server 201, a plurality of base stations (202,
203), and nodes (204, 205, 206). Transmission data from nodes are
transmitted wirelessly to the base stations and each base station
transfers the received data to the management server 201. The
management server 2 transfers the data received from the base
station to a Web site on the Internet.
[0048] In the case of node initiative wireless communication
protocol switching, any node can select either ALOHA or CDMA as the
protocol to be applied for communication with its associated base
station and start communication in accordance with the selected
protocol. Therefore, there is no overhead by the protocol switching
in this case.
[0049] In order to directly apply the index value Ef expressed by
Equation (1) to the node initiative protocol switching, the base
station has to notify the nodes of information as to the index
value Ef so that each node can recognize the index value Ef. This
is because each node cannot obtain the information on the system
throughput by itself. In this case, overhead due to the
communication of the throughput-related information occurs between
the base station and the nodes. This results in a disadvantage such
that overhead adversely affects the index value Ef.
[0050] Therefore, in the first embodiment, some parameter should be
used to evaluate environmental communication circumstances more
straightforwardly as compared with the evaluation according to
Equation (1). As such parameter, for example, packet retransmit
count at each node, the number of times (busy count) of detecting
another node in communication detected by carrier sense, etc. may
be considered applicable. The packet retransmit count is a
parameter value that each node can count by itself. Likewise, the
busy count detected by carrier sense is a parameter value that each
node can measure by itself without the help from the base station.
Since these parameter values can be measured at the base station
side, base station initiative protocol switching is also
possible.
[0051] Here, an example in which packet retransmit count measured
at each node is used as a measure of evaluating the throughput per
power consumption indicated by Equation (1) will be discussed.
Packet retransmit count is proportional to packet loss. As the
reasons of packet loss occurrence, conceivable two reasons are
propagation loss due to that the node of interest is far from the
base station, and loss caused by collision of packets transmitted
from two or more nodes simultaneously.
[0052] The rate at which the former packet loss is likely to occur
is constant because this loss is independent of the number of other
nodes existing around the node of interest. However, the latter
packet loss varies over time because it is dependent on the number
of other nodes existing around the node of interest. Therefore, by
monitoring a temporal variation in the packet retransmit count, the
cause of packet loss can be conjectured. For example, when a node
operating in accordance with the ALOHA method has detected the
increase in packet loss due to packet collision, the system
efficiency can be improved by switching the protocol to the
CSMA.
[0053] FIG. 9 shows a structural diagram of a node (204 to
206).
[0054] The node comprises a wireless unit (Radio Frequency unit)
901, a wireless communication protocol selecting unit (RCMSD) 902,
a database unit 903, and a communication processing unit (MEP) 904.
The wireless unit 901 transmits and receives data to and from the
associated base station according to a wireless communication
protocol selected by the wireless communication protocol selecting
unit 902. In the wireless communication protocol selecting unit
902, a control program for allowing the wireless unit 901 and the
wireless processing unit 904 to operate selectively under either of
ALOHA and CDMA. The communication processing unit 904 issues a
protocol switching (program switching) instruction to the wireless
communication protocol selecting unit 902 based on received
information and packet retransmit count (the number of times of
retransmission within a predetermined period) from the wireless
unit 901 and protocol switching conditions or the like stored in
the database unit 903. In the database unit 903, the switching
conditions for switching, for example, from ALOHA to CSMA when
packet retransmit frequency has exceeded a threshold are stored,
for example, as the correspondence of the retransmit counts with
the available protocols. The communication processing unit 904 can
determine a wireless communication protocol to be applied by
measuring the packet retransmit frequency and referring to the
database.
[0055] FIG. 12 shows a flowchart of basic operation of a
conventional node according to the CSMA in a mobile communication
system having three cells repeat pattern arrangement.
[0056] The node enters a standby state at its power-on (1201) and
is activated periodically at regular time intervals (1202). The
node selects one of frequencies of three available channels (1203)
and judges whether data transmission is possible or not, by
carrying out a carrier sense to confirm the status of data
transmission by the other nodes (1204). As a result of the carrier
sense, if a busy state was detected while another node is
transmitting data, the node waits for an elapse of random time
(1209). After that, the node judges again whether data transmission
is possible or not (1204).
[0057] If it was judged that none of the other nodes is
transmitting data, the node performs wireless data transmission
(1205), and waits for receiving an ACK packet transmitted from the
base station in response to the transmission packet (1206). If no
ACK packet was received within a predetermined time, the node
increments the packet retransmit count. If the retransmit count is
less than or equal to a predetermined threshold value N, the node
judges again whether data transmission is possible or not (1204).
If the retransmit count has exceeded the predetermined threshold
value N, the node changes the frequency to be applied at Step 1203
and judges whether data transmission is possible or not (1204).
When an ACK packet was received within the predetermined time, the
node returns to the standby mode (1201) because the data
transmission from the node to the base station has been
completed.
[0058] Basic operation flow of the node according to the ALOHA is
one that excludes the step (1204) of determining whether data
transmission is possible or not by the carrier sense and the step
(1209) of waiting when the busy state is detected, from the
flowchart of FIG. 12.
[0059] Next, basic operation of a node in the first embodiment of
the invention will be described with reference to FIG. 14. The node
enters a standby state at its power-on (1401) and is activated
periodically at regular time intervals (1402). The node selects one
of frequencies of three available channels (1403) and judges
whether data transmission is possible or not by carrying out a
carrier sense to confirm the status of data transmission by the
other nodes (1404). As a result of the carrier sense, if it is in
the busy state, the node waits for an elapse of random time (1409).
In the present embodiment, after the elapse of random time, the
node increments the count of waiting in the busy state (1410) and
returns to Step 1404 of determining whether data transmission is
possible or not.
[0060] If it was judged that none of the other nodes is
transmitting data, the node performs wireless data transmission
(1405) and waits for receiving an ACK packet from the base station
(1406). If no ACK packet was received within a predetermined time,
the node increments the packet retransmit count (1408). If the
retransmit count is less than or equal to a predetermined threshold
value N, the node judges again whether data transmission is
possible or not (1404). If the retransmit count has exceeded the
threshold value N, the node clears the carrier sense induced wait
count which has been counted until now (1411), changes the
frequency to be used at Step 1403, and judges whether data
transmission is possible or not (1404).
[0061] When an ACK packet was received, the node is placed in the
standby mode (1401) because the data transmission from the node to
the base station has been completed. At this time, the
communication processing unit compares the retransmit count for
data transmission as well as the carrier sense induced wait count
with predetermined values, respectively (1412, 1413) and issues a
protocol switching instruction to the wireless communication
protocol selecting unit (1414, 1415).
[0062] The CSMA method generally offers a better throughput
performance under various environments than the ALOHA method.
However, since the ALOHA method does not perform carrier sense
control, it provides a better system efficiency than the CSMA in
terms of power consumption. In the case where the number of nodes
is small, there is no significant difference in throughput
performance between the ALOHA method and CSMA method. As the number
of nodes increases, packet loss increases and throughput decreases
in the ALOHA method. There is also a possibility that power
consumption due to an increased number of packet retransmissions
becomes more than the saved power estimated by omitting the carrier
sense control.
[0063] Thus, in order to improve the system efficiency, it is
advisable to switch from the ALOHA to the CSMA when:
[0064] "Power consumption by carrier sense control<Power
consumption by packet retransmissions"
[0065] The communication environment may be improved during
communication under the CSMA and, conversely, there is a
possibility that power consumption due to packet retransmissions
occurring in the ALOHA becomes less than the power consumption for
the carrier sense control.
[0066] "Power consumption by carrier sense control>Power
consumption by packet retransmissions"
[0067] In this case, it is advisable to reversely switch from the
CSMA method to the ALOHA method so that the system efficiency can
be improved. In order to realize such protocol switching,
therefore, control may be carried out as follows.
[0068] The communication processing unit 704 reads out parameters
for calculation from the database unit 703 and calculates the power
consumptions due to the carrier sense control and the packet
retransmissions, from these parameters, the carrier sense count,
and the retransmit count. Based on the calculation result, the
communication processing unit 704 issues a protocol switching
instruction to the wireless communication protocol selecting unit
702. The wireless communication protocol selecting unit selects a
program corresponding to the protocol specified by the instruction,
from the previously prepared communication protocol programs for
CSMA and ALOHA.
[0069] FIG. 10 shows a structural diagram of the base station (202,
203). The base station is comprised of a wireless unit (RF) 1001, a
wireless communication protocol selecting unit (RCMSD) 1002, a
communication processing unit (MEP) 1004, a database unit 1003, and
a wired interface 1005.
[0070] FIG. 13 shows a flowchart of basic operation of the base
station.
[0071] The base station is always in a ready to receive state
(1301). Upon receiving a packet from a node (1302), the base
station checks a Cyclic Redundancy Check (CRC) of the received
packet to determine whether the reception is successful (1303).
When a CRC error occurs, the packet is discarded. If no error
occurs in the CRC, the base station transmits an ACK packet to the
source node of the packet (1304) and transmits the received data to
the management server through the wired interface (1305). In the
case of node initiative applicable protocol switching (between
ALOHA and CSMA), there is no need for the base station side to
perform protocol switching.
[0072] FIG. 11 shows a structure of a management server 201.
[0073] The management server is provided with a wired interface
1101 and a database unit 1103. Upon receiving data from a base
station through the wired interface 1101, the management server
stores the received data into the database unit 1003 or transmits
the received data to another device connected via the network.
[0074] In the case of node initiative wireless communication
protocol switching, as illustrated in the present embodiment,
applicable protocols are limited to those that are switchable by
each node in an autonomous manner, such as ALOHA and CSMA.
[0075] Although packet retransmit count is used as the index to
determine protocol switching in the foregoing description, indices
other than the packet retransmit count can be used if they are
parameters with which the amount of data transfer per power
consumption and the number of nodes under the management of a base
station are derived directly or indirectly. For example, Receive
Signal Strength Index (RSSI), Bit Error Rate (BER), Packet Error
Rate (PER), a ratio of failed transmission count to successful
transmission count, etc. can be used as the index. Each value of
BER and PER is calculated by a calculating formula according to a
wireless communication method for a physical layer.
EMBODIMENT 2
[0076] As a second embodiment, base station initiative protocol
switching will be described. In the case of base station initiative
protocol switching, more protocol types are applicable. For
example, wireless communication protocols such as TDMA, BTMA, and
ISMA methods, in which each node transmits packets in accordance
with an instruction or synchronization information from the base
station, can be added to selectable protocols.
[0077] In this case, for protocol switching, the base station has
to send a protocol switching instruction to each node. Accordingly,
overhead for communication for this purpose occurs, but the system
can achieve enhanced control as a whole because it becomes possible
to handle statistic information in the whole system in the case of
base station initiative.
[0078] In similar to the case for the first embodiment, the base
station in the present embodiment comprises a wireless unit (RF)
1001, a wireless communication protocol selecting unit (RCMSD)
1002, a database unit 1003, a communication processing unit (MEP)
1004, and a wired interface 1005, as shown in FIG. 10. The wireless
unit 1001 transmits and receives data in accordance with a program
for a wireless communication protocol which is currently selected
from among the programs corresponding to the wireless communication
protocols provided in the wireless communication protocol selecting
unit 1002.
[0079] In the wireless communication protocol selecting unit 1002,
a plurality of protocol programs for selectively operating the
wireless unit 1001 and the wireless processing unit 1004 in
accordance with one of different wireless communication protocols
are prepared. The communication processing unit 1004 issues a
protocol switching (program switching) instruction to the wireless
communication protocol selecting unit 1002 based on received
information and the number of nodes under management of the base
station as well as protocol switching conditions or the like stored
in the database unit 1003. In the database unit 1003, the wireless
communication protocol switching conditions or the like are stored,
and the communication processing unit 1004 can determine a protocol
to be applied by referring to the database unit 1003.
[0080] A flowchart illustrating basic operation of a node in the
present embodiment is shown in FIG. 15 and a flowchart illustrating
basic operation of the base station is shown in FIG. 16.
[0081] As shown in FIG. 15, each node is in the standby state
(1501), and activated periodically at regular time intervals
(1502). The node selects one of the frequencies of three available
channels (1503) and attempts to receive a pilot signal transmitted
from the base station (1504). Upon receiving a pilot signal, the
node selects a protocol specified by the received pilot signal
(1505), transmits data (1506), shuts down the power (1507), and
returns to the standby state again.
[0082] Here, data transmission (1506) is performed in accordance
with the selected protocol. The base station can make use of the
amount of data transfer per power consumption, expressed by
Equation (1), as a measure of the efficiency of a wireless
communication protocol.
[0083] FIG. 6 shows simulation results of the information transfer
efficiencies of ALOHA (601), CSMA (602), and TDMA (603), with the
number of nodes managed by the base station plotted on the abscissa
and the index value Ef according to Equation (1) plotted on the
ordinate. Based on the evaluation result, it is possible to
determine an optimum wireless communication protocol in accordance
with the number of nodes under management of the base station, and
the number of nodes to be used as a threshold for protocol
switching, thereby to create a table for protocol switching.
[0084] Examples of tables for protocol switching are shown in FIG.
7 and FIG. 8.
[0085] FIG. 7 shows a table for applicable protocol switching
according to the number of nodes. Based on the simulation results
of FIG. 6, this protocol switching table indicates that, within the
range up to 1000 nodes, the access method to be selected is the
ALOHA if the number of nodes falls between 0 and 250, the CSMA if
the number of nodes falls between 250 and 550, and the TDMA if the
number of nodes falls between 550 and 1000.
[0086] FIG. 8 shows a table for wireless communication protocol
switching according to the protocol currently applied, the number
of nodes, the index Ef of throughput per power consumption. Based
on the simulation results of FIG. 6, this table indicates that,
within the range up to 1000 nodes and when the currently applied
protocol is the ALOHA, the ALOHA is maintained if the number of
nodes falls between 0 and 250 and the Ef value falls between
1.85E-4 and 1.7E-4, the protocol is switched to the CSMA if the
number of nodes falls between 250 and 550 and the Ef value falls
between 1.7E4 and 1.5E-4, and the protocol is switched to the TDMA
if the number of nodes falls between 550 and 1000 and the Ef value
is below 1.5E-4.
[0087] In a similar manner, when the protocol currently applied is
the CSMA or the TDMA, as well, a wireless communication protocol to
be selected is specified according to the number of nodes and the
Ef value. By referring to this table, therefore, it is able to
determine whether the currently used protocol should be switched to
another protocol and what protocol should be selected.
[0088] At a base station, the communication processing unit 1004
stores a terminal ID notified from each node in the database unit
1003 in order to recognize the number of nodes under its
management. In the database unit 1003, the protocol switching table
shown in FIG. 7 is retained beforehand. Depending on the number of
nodes known from the terminal IDs, the communication processing
unit 1004 determines from the above protocol switching table, an
optimum wireless communication protocol to be applied for
communication with the nodes.
[0089] The communication processing unit 1004 notifies the wireless
communication protocol selecting unit 1002 of the determined
wireless communication protocol. A protocol switching instruction
from the base station to each node may be issued, for example, by
setting information instructive protocol switching in an ACK packet
to be returned when a node accesses the base station. At each node
having received the ACK packet, the communication processing unit
904 analyzes the protocol switching instruction and issues a
switching instruction to the wireless communication protocol
selecting unit 902 so as to select a protocol in accordance with
the instruction from the base station.
[0090] In the TDMA method, the base station always transmits a
Pilot signal necessary for synchronization. Thus, in the case of
switching the current protocol to the TDMA, the operation of the
base station is switched to the TDMA mode for transmitting the
Pilot signal so that each node can establish synchronization by
receiving the Pilot signal, and thereafter transmit and receive
packets in synchronization with the base station. The management
server 201 does not need to execute specific operation with respect
to the present embodiment, in similar to the case for the first
embodiment.
[0091] As other evaluation methods for directly or indirectly
deriving the amount of information transfer per power consumption
and the number of nodes under management of the base station,
following methods are exemplified. Although following methods have
differences in detecting operations and evaluation methods for
determining protocol switching, a protocol switching instruction
from the base station to each node may be issued in the same manner
as described above.
[0092] At the base station, the communication processing unit 802
acquires, based on the packets received from each node, data
representing packet transmit counts (including retransmit counts)
at each node, packet receive counts, and the total sum of packets
received by the base station. Based on the data, the communication
processing unit 802 calculates, in real time, the overall power
consumption for all nodes and the overall throughput for all nodes
as expressed in Equation (1). For example, these values can be
calculated as follows: "Overall throughput for all nodes=the total
sum of packets received by the base station/.SIGMA. transmit
counts" "Overall power consumption for all nodes=transmitting
power.times..SIGMA. transmit counts+receiving power.times..SIGMA.
receive counts"
[0093] From the value Ef of Equation (1) and the number of nodes
under the management of the base station, the base station
determines a wireless communication protocol to be applied,
according to the table which is shown in FIG. 8 and retained
beforehand in the database unit 1008. If protocol switching is
required, the base station gives to each node an instruction for
switching to the selected wireless communication protocol by using
the pilot signal or ACK packet. Then, the communication processing
unit 1002 of the base station acquires various parameters (transmit
counts, receive counts) and stores them into the database unit
1003.
[0094] Next, an example will be described on a case where the base
station performs wireless communication protocol switching based on
BER. The communication processing unit 1002 calculates the S/N
ratio based on the transmitting power at each node extracted from
received packet data and the radio signal receive level at the base
station acquired from the RF unit 1001. A calculating formula for
BER is derived beforehand from Equation (2) for each communication
protocol. The communication processing unit 1002 calculates the
value of BER based on this calculating formula, selects an optimum
wireless communication protocol when the BER exceeds a
predetermined threshold value, and issues a protocol switching
instruction to the wireless communication protocol selecting unit
1004 and each node. Erfc .function. ( x ) = 2 .pi. .times. .intg. x
.infin. .times. exp .function. ( - .mu. 2 ) .times. .times. d u ( 2
) ##EQU1##
[0095] Next, an example will be described on a case of evaluation
using PER. In the case of wireless communication protocol switching
based on PER, the PER can be obtained from the BER and packet
length. Accordingly, a calculating formula for PER is derived
beforehand. The communication processing unit 1002 can obtain the
value of PER by calculating a signal power to noise power ratio
(S/N) based on the transmitting power at each node extracted from
packet data received from the node and the radio signal receive
level at the base station acquired from the RF unit 1001, in the
same manner as the case for the BER. The communication processing
unit 1002 selects a wireless communication protocol when the PER
exceeds a predetermined threshold, and issues a protocol switching
instruction to the wireless communication protocol selecting unit
1004 and each node.
EMBODIMENT 3
[0096] As a third embodiment of the invention, management server
initiative wireless communication protocol switching will be
described. In the case of management server initiative wireless
communication protocol switching, the load on a base station can be
reduced. The flowcharts of the basic operations of a node, base
station, and management server in the present embodiment are shown
in FIG. 17, FIG. 18, and FIG. 19, respectively.
[0097] At the base station, the communication processing unit 1002
acquires data representing packet transmit counts (including
retransmit counts) and packet receive counts at each node and the
total sum of received packets at the base station and sends these
data to the management server through the interface 1005. The
management server receives these data by the interface 1101 and the
communication processing unit 1102 calculates, in real time, the
overall power consumption for all nodes and the overall throughput
for all nodes as expressed in Equation (1), based on the received
data. For example, these can be calculated as follows: "Overall
throughput for all nodes=the total sum of packets received/.SIGMA.
transmit counts" "Overall power consumption for all
nodes=transmitting power.times..SIGMA. transmit counts+receiving
power.times..SIGMA. receive counts"
[0098] From the value of Equation (1) and the number of nodes
managed by the base station, the management server switches to a
wireless communication protocol based on the results shown in FIG.
6. In similar to the case for Embodiment 2, the management server
retains beforehand the results of FIG. 6 in a form of a table in
the database unit 1003. Then, the management server notifies the
base station of the selected wireless communication protocol as a
notification signal through the interface 1101.
[0099] When the base station receives the notification of the
wireless communication protocol through the interface 1005, the
wireless communication protocol selecting unit 1002 selects a
program corresponding to the above wireless communication protocol
and performs wireless communication protocol switching by
activating that program. In the same manner as noted in Embodiment
2, the base station notifies each node of the protocol
switching.
[0100] As evaluation methods for directly or indirectly deriving
the amount of data transfer per power consumption and the number of
nodes under management of the base station, for example, Bit Error
Rates (BER) or Packet Error Rates (PER) for the communication
between the base station and the nodes may be used.
INDUSTRIAL APPLICABILITY
[0101] By dynamically switching to a wireless communication
protocol adaptively in accordance with the change in communication
environmental, the wireless communication system according to the
present invention can realize wireless communication with higher
system throughput and lower power consumption.
* * * * *